Here is a GOTTA have gadget if you are looking for accessories/USB/Monitoring

I found these on ebay and amazon about a year ago, they come in a couple different flavors, i mostly bought the 2 or 3 gang version... they easilyunscrew and you can fill the holes with whatever you need.... I installed about 4-5 units through-out the coach for the USB and maybe the Lighter Pug. The dash I have a dual unit with the gauge and the USB.....

Although rated at 12V, the gauge appears to work fine up on my 24V side and reads the dash voltage digitally for quick easy reference of 27.2 running down the road! An inexpensive item that is a breeze to install and provides some GREAT functionality. available in RED or Blue Gauge and the USB has a low amp and high amp side... and all are working flawless a year later ( go figure cheap made in China, had my doubts lol)